Thanks for your reply, however I must admit that it doesn't "feel" right to me. As I mentioned, both the motor and gearbox can be turned quite freely when apart, there is no great friction. It is only when brought together that they're locked solid, just a little play (backlash). It feels very similar to my attempt to turn a worm gear in reverse.
If I had to guess I'd say that when driven normally the planetary gears are pressed against the outer (annular) ring and turn freely, but when driven in reverse the planetary gears squeeze together onto the central (sun) gear, past the required tooth clearance, and hence it locks up instead of turning - and since the sun gear is on the motor spindle, this doesn't happen when motor and gearbox are apart. This is a pure guess like I said, but that's what it feels like.